April was the first month in recorded history with average carbon dioxide levels at or above 400 parts per million across the Northern Hemisphere, according to a May 26 announcement by the World Meteorological Organization.

Climate scientists first recorded the troubling peak in the greenhouse gas in the Arctic in 2012 and in Hawaii last year (SN: 12/28/2013, p. 26), but the rest of the globe had yet to consistently hit the high mark. The 400 ppm level is largely symbolic, representing nearly 150 percent of the CO2 levels of pre-industrial times.

Although scientists expect CO2 levels to continue rising, the Northern Hemisphere’s monthlong record for the greenhouse gas should sound an alarm for addressing emissions and climate change, the World Meteorological Organization said. From 2002 to 2012, CO2 was responsible for 85 percent of the increase in the atmosphere’s heat-trapping ability.